1. Field of the Invention
Various embodiments of the present invention relate to a plasma electron source apparatus, a method for generating pulsed electron beams and the use of a plasma electron source apparatus.
2. Description of Related Art
Freeform fabrication or additive manufacturing is a method for forming three-dimensional articles through successive fusion of chosen parts of powder layers applied to a worktable. A method and apparatus according to this technique is disclosed in US Patent Application Publication No. 2009/0152771.
Such an apparatus may comprise a work table on which the three-dimensional article is to be formed, a powder dispenser, arranged to lay down a thin layer of powder on the work table for the formation of a powder bed, a ray gun for delivering energy to the powder whereby fusion of the powder takes place, elements for control of the ray given off by the ray gun over the powder bed for the formation of a cross section of the three-dimensional article through fusion of parts of the powder bed, and a controlling computer, in which information is stored concerning consecutive cross sections of the three-dimensional article. A three-dimensional article is formed through consecutive fusions of consecutively formed cross sections of powder layers, successively laid down by the powder dispenser.
When using an electron beam for delivering energy to the powder in additive manufacturing it is desirable to switch on and off the electron beam relatively fast in order to control the heat distribution and the fusion mechanism.
The switching frequency is limited by the temperature time constant for the filament material when an electron beam is generated from a heated metal filament. It is difficult for not saying impossible to switch the electron generation from a hot filament on and off faster than 50 Hz.
The switching frequency of hot filament electron generation may be increased by using a so called triod design. In the triod design a grid is used, which rapidly may switch on and off the electron beam. However, with a triod design it may be difficult to form a usable electron beam for freeform fabrication with good quality with respect to focus and intensity stability over space and time and for variable current.
When an electron beam is generated from a plasma source the switching frequency is limited by the time for creating a plasma and extinguish a plasma. It is also limited by the switching time for the high voltage needed for turning on and off the plasma. The highest possible plasma generating frequency is in theory about 10 MHz, but in practice limited by the switching time of the high voltage drive electronics.
The known methods for creating pulsed electron beams do not have a sufficiently high pulse rate without influencing electron beam quality, which may be a problem in additive manufacturing.